This investigator hopes to have a career examining psychopharmacologic and behavioral interventions for preschool aged youth with disruptive behavioral disorders. This award would allow the investigator to pursue necessary additional training to gain research skills and experience in preparation for a K award application, which will be instrumental to this applicant's eventual goals of becoming an independent, successful, clinical child psychiatry researcher. In a pilot study under a training grant to this investigator, preschool three-to-five year olds rigorously diagnosed with Attention Deficit/Hyperactivity Disorder (ADHD) will be recruited from a general child psychiatry clinic to enroll into a small (approximately 30 children), 6 week double blind, crossover comparison trial of extended release mixed amphetamine salts (XR-MAS) versus placebo to study efficacy and safety tolerance. The specific aim is to evaluate safety and efficacy data of XR-MAS versus placebo in children who will experience both medications in a brief study period. The recently completed NIMH-funded multisite Preschool ADHD Treatment Study (PATS) demonstrates that short-acting MPH can be safe and effective in reducing core symptoms of ADHD, but there is no data for providers to consider initiating MAS versus MPH. Recent pharmacoepidemiological data indicates prescribers are increasingly using both types of stimulants in this population, though there is a dearth of controlled studies evaluating these medications in preschool children. It is proposed that XR-MAS will have a favorable safety and efficacy profile, and that this pilot study, followed by a larger K award study, could demonstrate that under short term study conditions, XR-MAS are reasonably well tolerated and effective in particularly young and developmentally vulnerable children at risk for poor educational, social and physical outcomes. The importance of demonstrating and comparing XR-MAS versus placebo in this population is due to ADHD being a public health burden amongst preschool children as indicated in recent reports this population is 3 to 5 times more likely to be expelled from daycare settings as adolescents are from high school. Additionally, this study and the training proposal will help this investigator become an early career scientist who can investigate the short and long term risks and benefits regarding the use of psychotropic medication in developmentally vulnerable children. The data from this small study will be used in a K award application to further investigate strategies to minimize symptom burden and functional impairment in this vulnerable population.